The following description makes specific reference to axisymmetric cylindrical systems using solenoidal magnet arrangements. The terms “radial”, “axial” and the like should be interpreted accordingly, unless the context requires otherwise.
The size and shape of the imaging volume of an MRI imaging system is an important driver in the design and cost of a magnet for such a system. The size and shape of the imaging volume also known as the field-of-view (FOV) can be defined by a homogeneity contour, such as the 3 ppm pk-pk contour, depending on the type of imaging required. The required radial diameter of the imaging volume is often determined by the requirement for shoulder and breast imaging. To achieve imaging volumes with sufficient radial width, magnet designers typically target a radially distended, oblate ellipsoidal imaging volume in their design.
However, there is also a demand for imaging volumes of a cylindrical shape, which are advantageous in the imaging of large areas, such as the abdomen. It is presently known to effectively achieve such cylindrical imaging volumes by joining two images each derived from an oblate ellipsoidal imaging volume, taken with the patient moved along the axis of the magnet, in the post processing stage of the imaging procedure. The use of a substantially cylindrical imaging region has the advantage that, for a given volume of the FOV, a composite image of the human abdominal region may be formed with less patient table moves than with a FOV of more conventional elliptical shape.
FIG. 1 shows example cross-sections of a presently achievable oblate ellipsoid imaging volume 10, and a presently achievable cylindrical imaging region 12 of the same volume, each being defined lying within an appropriate homogeneity contour. Note that the patient is aligned with the spine parallel to the Z axis 16 of the magnet. The ellipsoidal imaging volume 10 has an axial length z of approximately 35 cm and a radial diameter x of about 44 cm. The cylindrical imaging region 12 has an axial length z of approximately 28 cm and a radial diameter x of about 40 cm.
A possible disadvantage of such a cylindrical FOV 12 is that human shoulder areas may at least partially fall outside of the cylindrical FOV, as can be appreciated by comparing FIGS. 2A and 2B, which show views of a toroidal volume 14, in which imaging is required to be possible, relative to a cylindrical FOV 12 and an elliptical FOV 10. This is a particular issue for magnets with a limited radial width of FOV.
When a shoulder image is required in an MRI system with a FOV of limited radial width, such as the cylindrical imaging volume 12 of FIGS. 1, 2, the homogeneity can be restored by a local, passive shim. This has been shown to work, but is cumbersome and requires manual intervention for mounting and removing the passive shim.
It has been found theoretically possible to arrange for electrically operated shim coils to be switchable to convert a nominal ellipsoidal imaging volume 10, capable of imaging in toroidal volume, into a cylindrical imaging volume 12 with the same volume as the ellipsoidal imaging volume 10 and useful for imaging the abdomen. However, as illustrated in FIG. 1, this will result in a cylindrical imaging volume 12 with less radial width x than the equivalent ellipsoid volume 10. The ellipsoidal imaging volume 10, and the magnet to produce it, is typically designed to have a maximum radial diameter x sufficient to accommodate a patient's shoulders. As the cylindrical volume 12, adapted from the ellipsoid volume 10, has a reduced radial diameter x, the cylindrical volume will miss a significant part of the shoulder volume which is included within the ellipsoid volume 10. Although theoretically possible, the shim coils required to achieve this field adjustment have been found to be impractically large.
The present invention aims to address the above-mentioned difficulties, to provide a practical arrangement for generating an imaging volume of increased maximum radial diameter for a given MRI imaging system, by adaptation of a nominally substantially cylindrical imaging region.
An active, resistive shim coil has been considered which would change the imaging volume from a nominally cylindrical volume to an elliptical one suitable for imaging the toroidal volume, but such a shim is considered to be impractical, because of the amount of electrical power required. A resistive shim which specifically targets the toroidal volume, by deformation of a nominally substantially cylindrical imaging region is shown to be practical, according to the present invention. Such a shim coil will be referred to herein as a toroidal shim.
Accordingly, the present invention provides arrangements and methods as defined in the appended claims.